Cells of various shapes and sizes are used in battery systems for many electronic apparatus. There is a desire to make the size smaller, the weight lighter or the operating time longer, with a higher capacity of the cells.
For the cells limited in size by international standards, an effective means to make the capacity high is to have the inner volume enlarged with wasted space reduced to increase the active materials for the positive and negative electrodes. On the other hand, cells must have high reliability, durability, and be leak-proof in severe circumstances including very high or low temperature, in which the electronic watches, clocks or semiconductor memories are used.
FIGS. 6, 7, and 8, show a prior art zinc-air alkaline button-type cell for hearing aid. The cell comprises a cup-shaped negative electrode case 31, having therein a negative electrode 34 consisting mainly of amalgamated zinc powder. The negative electrode case 31 is surrounded by a cup-shaped positive electrode case 33 of nickel-plated steel sheet, with a generally L-shaped insulating gasket 32 of polyamide resin in between. The open edge of the positive electrode case 33 is bent inward to press the gasket 32 to the negative electrode case 31 to finish the sealed housing. Usually, the negative electrode case 31 is formed of clad or plated sheet with stainless steel 31c as the core, and a copper layer 31e to contact with the negative electrode inside the cell and a nickel layer 31d to function as the negative electrode terminal as shown in FIG. 7. The flat bottom of the positive electrode case 33 has one to three air-holes covered by a diffusion paper 36 on the inside surface. The diffusion paper 36 is porous and enables the oxygen, which is the positive active material, to diffuse uniformly from the atmosphere to the positive electrode 37 through a hydrophobic membrane 38. Stacked on the diffusion paper 36 are a hydrophobic membrane 38, the positive electrode 37, and a separator 39, each being a disc with the same diameter as the inside diameter of the positive electrode case 33. The positive electrode 37, generally called an air electrode, is prepared by mixing a catalyst consisting of manganese oxide and activated carbon, and electroconducting acetylene black, with further addition of polytetrafluorethylene (PTFE) dispersion, applying the mixture on a metallic screen, and drying. The separator 39 consists of a micro-porous polypropylene (PP) membrane and an electrolyte-absorbing sheet.
When the cell is sealed, the circumference of the stacked hydrophobic membrane 38, positive electrode 37 and separator 39 are pressed to the inside bottom of positive electrode case 33 with the bottom part of the gasket 32 in between, to prevent leakage. This structure, with a large volume in the negative electrode case 31, is suitable for high capacity. However, its leak-proof properties were not satisfactory, because use of strong seal-pressure for sufficient leak-proof properties forced the side wall of the negative electrode case 31 with its sharp under-edge 31a into the upper surface of the gasket 32 often cutting it to destroy electrical insulation. In the past, the zinc-air cells air holes 35 have been closed with an air-tight sealing tape to prevent deterioration of the cell by drying up or invasion of moisture from the atmosphere. For example, a zinc-air alkaline button cell of type R44 (11.6 mm diameter, 5.4 mm overall height) had a capacity 2.1 times compared with the mercury cell of the same size, but was not always reliable due to its low leak-resistance and poor storability. To improve the leak-proof properties, a sealant 30 of polyethylene sulfonide or butyl rubber was placed between the negative electrode case 31 and the gasket 32, as shown in FIG. 7, but without much success.
Referring to FIG. 8, a more improved type zinc-air alkaline button cell comprises a negative electrode case 41, gasket 42, positive electrode case 43, negative electrode 44, air hole 45, diffusion paper 46, positive electrode 47, hydrophobic membrane 48, and separator 49 of the same materials as the parts in the cells described previously in reference to FIG. 6 and FIG. 7. The cell of FIG. 8 is different from that in FIG. 6 in that the negative electrode case 41 has the side wall folded back to form a round edge 41a. With such a negative electrode case 41, the gasket 42 is not cut from the upper side when the negative electrode case is subjected to high pressure. However, the two-layered side-wall of the negative electrode case 41 resulted in a smaller inside-diameter of the case and smaller inside volume and thus smaller electrical capacity. To cope with such shortcomings and to obtain higher capacity, attempts were made to make the thickness of the negative electrode case 41, gasket 42, and positive electrode case 43 thinner. However, the difficulty of preparing thinner parts and of obtaining sufficient sealing strength presented a limit to this endeavour and the decrease of capacity was not fully recovered.
As for the gasket with an L-shaped section, it functions in the assembly of a cell by surrounding the side wall of the negative electrode case with a sealant in between the negative electrode case and the positive electrode case. The gasket of polyamide resin, which is used in most of the flat-type cells especially those including alkaline electrolyte, is apt to change its form and size, by absorbing moisture. Therefore, usually, the negative electrode case and the polyamide resin gasket are assembled to form one body by drying and quick application during assembly of the cell. If not, the gasket is deformed due to moisture absorption thus generating a gap between the upper surface of the gasket and the lower end of the negative electrode case wall. If a cell is sealed with a negative electrode case in this condition, the air in the gap between the negative electrode case and the gasket is forced to contact the electrolyte sealed between the positive electrode and the negative electrode, to cause deterioration in the leak-proof characteristic of the cell.
It is a goal of the present invention to provide a flat-type cell which has a negative electrode case with a large inside volume, and is highly leak-proof, reliable and of high capacity.